generation of high-order harmonics with ultra-short pulses from filamentation

Generation of high-order harmonicswith ultra-short pulses from

filamentation

Daniel S. Steingrube1,2#�, Emilia Schulz1,2#, Thomas Binhammer3,Tobias Vockerodt1,2, Uwe Morgner1,2,4, and Milutin Kovacev1,2

1Leibniz Universitat Hannover, Institut fur Quantenoptik, Welfengarten 1, D-30167 Hannover,Germany

2QUEST, Centre for Quantum Engineering and Space-Time Research, Welfengarten 1,D-30167 Hannover, Germany

3VENTEON Laser Technologies GmbH, D-30167 Hannover, Germany4Laser Zentrum Hannover e.V., Hollerithallee 8, D-30419 Hannover, Germany

#Equal [email protected]

Abstract: 7-fs-pulses with 0.3 mJ are obtained after filamentation inargon and compression by double-chirped-mirrors. These pulses are usedto generate high-order harmonics in a semi-infinite gas cell in differentnoble gases. Spectral broadening of high-order harmonics in xenon andargon is observed. In neon, an extended continuous cut-off region downto 10 nm (124 eV) is observed which is to the best of our knowledge thehighest cut-off energy obtained by filamented pulses. Our result suggeststhe feasibility of single attosecond-pulse-generation at both high photonflux and high cut-off energy.

© 2009 Optical Society of America

OCIS codes: (020.4180) Multiphoton processes; (190.0190) Nonlinear optics; (190.4160)Multiharmonic generation; (320.0320) Ultrafast optics; (320.5520) Pulse compression;(320.6629) Supercontinuum generation

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M. Uiberacker, A. Apolonski, F. Krausz, and R. Kienberger, “Intense 1.5-cycle near infrared laser waveforms andtheir use for the generation of ultra-broadband soft-x-ray harmonic continua,” New J. Phys. 9, 242 (2007).

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3. C. P. Hauri, W. Kornelis, F. W. Helbing, A. Couairon, A. Mysyrowicz, J. Biegert, and U. Keller, “Generation ofintense carrier-envelope phase-locked few-cycle laser pulses through filamentation,” Appl. Phys. B 79, 673–677(2004).

4. E. T. J. Nibbering, P. F. Curley, G. Grillon, B. S. Prade, M. A. Franco, F. Salin, and A. Mysyrowicz, “Conicalemission from self-guided femtosecond pulses in air,” Opt. Lett. 21, 62–65 (1996).

5. H. S. Chakraborty, M. B. Gaarde, and A. Couairon, “Single attosecond pulses from high harmonics driven byself-compressed filaments,” Opt. Lett. 31, 3662 (2006).

6. A. Couairon, H. S. Chakraborty, and M. B. Gaarde, “From single-cycle self-compressed filaments to isolatedattosecond pulses in noble gases,” Phys. Rev. A 77, 053814 (2008).

7. A. Zaır, A. Guandalini, F. Schapper, M. Holler, J. Biegert, L. Gallmann, A. Couairon, M. Franco, A. Mysyrowicz,and U. Keller, “Spatio-temporal characterization of few-cycle pulses obtained by filamentation,” Opt. Express15, 5394–5405 (2007).

8. M. Hentschel, R. Kienberger, C. Spielmann, G. A. Reider, N. Milosevic, T. Brabec, P. Corkum, U. Heinzmann,and M. Drescher, “Attosecond metrology,” Nature (London) 414, 509–513 (2001).

#109029 - $15.00 USD Received 20 Mar 2009; revised 24 Aug 2009; accepted 25 Aug 2009; published 27 Aug 2009

(C) 2009 OSA 31 August 2009 / Vol. 17, No. 18 / OPTICS EXPRESS 16177

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10. E. Schulz, T. Binhammer, D. S. Steingrube, S. Rausch, M. Kovacev, and U. Morgner, “Intense few-cycle laserpulses from self-compression in a self-guiding filament,” Appl. Phys. B 95, 269–272 (2009).

11. F. X. Kartner, U. Morgner, R. Ell, T. Schibli, J. G. Fujimoto, E. P. Ippen, V. Scheuer, G. Angelow, and T. Tschudi,“Ultrabroadband double-chirped mirror pairs for generation of octave spectra,” J. Opt. Soc. Am. B 18, 882–885(2001).

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15. D. S. Steingrube, T. Vockerodt, E. Schulz, U. Morgner, and M. Kovacev, “Phase-matching of high-order harmon-ics in a semi-infinite gas cell,” (2009). Submitted.

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1. Introduction

The emergence of isolated attosecond pulses opened up very new perspectives for the study ofelectron dynamics in atoms and molecules. While a train of attosecond pulses is emitted frommulti-cycle pulses during high-order harmonic generation (HHG), isolated attosecond pulsesare generated by few or single-cycle pulses [1]. Different setups have been realized to producefew-cycle infrared pulses, as for example spectral broadening and pulse re-compression in agas filled hollow fiber [2]. However, hollow fibers are limited by a maximal through-put pulseenergy. An alternative technique with promising energy scaling potential is based on non-lineareffects during the filamentation process in a cell filled with noble gases [3]. The realization isstraightforward, but at the output the spectral broadening behind the filament is not homoge-neous across the spatial beam profile. Due to conical emission and non-filamented parts of thebeam [4], the shortest pulse durations along the beam profile are only found in the white lightcore in the center of the beam profile, and care has to be taken to screen the outer parts. Basedon simulations [5, 6], it is then possible to generate isolated attosecond pulses by HHG. In [7]the generation of high-order harmonics up to 50 eV from filamented pulses was demonstrated.To the best of our knowledge, however, with few-cycle pulses from filaments HHG has neverreached the spectral regime beyond 90 eV where the generation of isolated attosecond pulseshas been demonstrated [8].

In the present paper, we report on HHG in a semi-infinite gas cell in different noble gasesusing compressed ultra-short pulses from a filament. High-order harmonics are generated inneon with a cut-off in the 10 nm-range, reproducible on a day-to-day basis. The spectral broad-ening of the harmonics and the spectral position of the cut-off is investigated by controlling thedispersion. Due to the potential energy scaling of the filamentation process by using gas withlow non-linear refractive index or more advanced techniques [9], this approach opens up newpossibilities for cut-off extension and high-energy single attosecond pulses.



filamentation cellpressure: 450 mbar

FM

4 %

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776nm, 30fs, 1.2mJ

R=-4000mm

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vacuum

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semi-infinite gas cell

Fig. 1. Sketch of filamentation and high-order harmonic generation setup. The inlet showsthe semi-infinite gas cell where the generated harmonics propagate through an abrupt tran-sition to vacuum realized by a self-drilled metal pinhole (see text). A monochromator setupwith multi-channel-plate detector records the harmonic spectra. A1, A2: apertures; FM:curved mirror; DCM1, DCM2: double chirped mirrors; L1: focusing lens.

2. Experimental setup

For HHG with pulses from filaments we use a chirped-pulse-amplification system (Dragon,KMLabs Inc.) delivering 30-fs-pulses centered at 776 nm with energies of 1.2 mJ at a repe-tition rate of 3 kHz. The system is not stabilized regarding the carrier-envelope phase. Thesepulses are spectrally broadened and subsequently compressed in a filamentation-setup shownin Fig. 1 [10]. With a curved silver mirror (R = −4000 mm) the pulses are focused into a 2 mlong gas cell filled with 450 mbar argon. Entrance and exit windows are 1 mm-CaF2-plates inBrewsters angle arrangements. In order to generate a stable single filament, an aperture (7 mmof diameter) is placed before the focusing-mirror transmitting about 0.85 mJ.

A second aperture (3 mm of diameter) is used behind the exit to select the white light core,and discriminate the non-filamented radiation and conical emission [6]. After the filament thepulses are focused via a 4 mm-thick (quartz)-lens with 500 mm focal length through a 2 mm-CaF2-window into the high-harmonic chamber. A pair of double-chirped mirrors (DCM) [11]compensating the influence of the lens and the entrance window allows for adjustment to zerochirp in the HHG-chamber. Zero chirp is realized by eight reflections at the DCM’s whichcorresponds to a group delay dispersion (GDD) of about −560 fs2. Our high-harmonic chamberis designed in a semi-infinite gas cell geometry (see inset in Fig. 1) [12, 13], where the entrancewindow is far away from the interaction region defined by the focal area. The gas cell is filledwith noble gases for HHG. Due to the large interaction range and the straight-forward setup,the semi-infinite gas cell is a promising tool for HHG with a high photon flux [14] without the



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Fig. 2. (a) Filament spectrum and phase after the DCM’s measured with SPIDER and (b)reconstruction of the corresponding 7.0-fs-pulse.

need for accurate alignment. An abrupt transition to vacuum for absorption-less propagation ofthe generated radiation is assured by a pinhole in a metal-plate used as a differential pumpingstage which is self-drilled by the laser before the experiment. We place the focus within the gascell about 1 cm in front of the pinhole for phase-matching of the cut-off-harmonics [15]. Thegenerated harmonic signal is spectrally resolved in a grazing incidence spectrometer (LHT 30,Horiba-Jobin-Yvon, 500 lines/mm).

3. Experimental results

The pulses after filament and DCM’s contain about 0.3 mJ of energy and are characterized witha SPIDER-setup [16] supporting the measurement of few-cycle pulses. The pulse spectrumwith a Fourier limit of 5.2 fs and the measured phase are shown in Fig. 2(a). From this datawe reconstruct an upper limit of 7.0 fs for the compressed pulse duration (Fig. 2(b)). Notethat dispersion compensation by the DCM’s is limited to wavelengths from 550 to 1200 nmexploiting not the full spectral range of the filamented pulse. Mirrors with advanced multi-layer optics designs [17] covering the full spectral bandwidth after the filament from 400 to900 nm [10] would provide potential for future optimization.

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Fig. 3. Harmonic spectra in (a) 5 mbar xenon, and (b) 13 mbar argon. For comparison,a harmonic spectrum in xenon generated by 30-fs-pulses directly from the amplifier withcomparable intensities is shown (dashed blue). Spectral broadening of the harmonics isobserved for the filamented pulses.

Using the characterized filament pulses, high-order harmonics are generated in different no-ble gases with different ionization potentials. Fig. 3(a) shows the harmonic spectrum in 5 mbarxenon for compressed filamented pulses. For comparison, a harmonic spectrum from non-



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Fig. 4. Harmonic spectra in 40 mbar neon. The spectra for positively, negatively, and chirp-compensated pulses are shown. Positive and negative chirp are realized by increasing anddecreasing the number of reflections at the DCM’s.

filamented 30-fs-pulses directly from the amplifier at comparable intensity is illustrated. Using7.0-fs-pulses from the filament, spectral broadening of the harmonics is clearly visible, whereasa discrete structure of the harmonics is observed with non-filamented pulses.

The harmonic spectra produced in argon are shown in Fig. 3(b). The spectral broadening ofthe harmonics is observable as well. Due to the higher ionization potential the cut-off extendsto smaller wavelengths.

We observed the broadest continuum at high cut-off energies in harmonic spectra generatedin neon. Fig. 4 shows the measured spectra obtained in 40 mbar neon. The high ionizationpotential of neon allows for a cut-off around 10 nm. Our pulses from the filamentation areshort enough to generate broadband continuum radiation. Modifying the re-compression setupwe can demonstrate the effect of longer pulses on the cut-off and the discrete nature of theharmonic emission. In order to elongate the pulses, we decrease the number of reflections atthe DCM’s by two for positive chirp with +140 fs2. Analogous, the number of reflections areincreased by two to achieve negatively chirped pulses. The spectra from longer pulses exhibita discrete peak structure with reduced cut-off and are shown in green and blue in Fig. 4. Thedecrease in the cut-off energy can be explained by a lower peak power for elongated pulses atconstant pulse energy.

4. Conclusion

In conclusion, we have shown high-order harmonics generated with ultra-short pulses from afilament. The results are well reproducible and stable. The observed pulse energy and durationresults in high-order harmonic generation in neon with a cut-off up to 10 nm corresponding to124 eV. This represents an extension of the HHG cut-off energy of more than 70 eV comparedto previous results [7] via filaments and proves the suitability of this scheme for the generationof high energy few-cycle pulses. The present setup is limited by the bandwidth of the DCM’s.Thus, optimizing the spectral properties of the DCM’s would allow for even shorter-pulses inthe single-cycle regime.

Single attosecond pulses via few-cycle-pulses from filamentation are now in reach employ-



ing multi-layer mirrors available in the EUV-spectral region filtering only the continuous partof the cut-off region. Due to to the promising energy scaling properties of the filamentation pro-cess higher photon and pulse energies in isolated attosecond pulses become feasible. Assuminga mirror covering 28 eV of bandwidth [18], our spectrum would correspond to an isolatedattosecond pulse with Fourier-limit below 80 as.

Acknowledgments

The authors like to thank Manfred Lein, and Thorsten Uphues for fruitful discussions. Thiswork was funded by Deutsche Forschungsgemeinschaft within the Cluster of ExcellenceQUEST, Centre for Quantum Engineering and Space-Time Research.



generation of high-order harmonics with ultra-short pulses from filamentation

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